No, do not go back and up! First thing, get the book "What Every Pianist Needs to Know About the Body" by Thomas Mark. You do seem to be too high and maybe sitting too far back on the piano bench. It's better not to say too much. Start with the book, and if you can get a good teacher - or even Alexander - that would be great.
Why would back and up do him any harm?
Because of all the harm that this kind of advice has done. And no, I was not thinking about my own personal experience. When I got the book that I mentioned, the author talks about the general advice that is given to piano students (and others) and the results from this. I recognized a fair bit of the things that I had followed, including the the end results. What I did not realize is how many people the wrong ideas of "posture" have affected. There should be NO external direction via the Internet telling people how they should sit, etc. That should be done with a teacher who knows what he or she is doing. The harm is tremendous. As one expert out there put it, "good posture" can be the most harmful at all - meaning the attempt to have it, and the strain it causes.In regards to what the OP needs to do: As long as he is sitting that high up, with his bench that close and sitting on the bench with little weight in the feet, trying to move his torso "back" and upright, is nothing but posing oneself, and creates an impossibility. If you try to reach for something that is lower, you have to stoop. If you aren't balanced in your feet, you'll strain your body otherwise. This should be done with an observing teacher, because trying to fix oneself from the outside is not a good idea. If not with an observing teacher, at least get the feeling of balance internally, but starting to understand how your body works (hence, the book I recommended.)
And what do you mean by "bench" and "seat"? A current goal of mine is to use the muscles on the lower hamstring as a sitting base, instead of the tailbone.